Many information storage systems include a disk shaped storage medium and a playback (read) and record (write) element. This read-write element (hereinafter xe2x80x9cheadxe2x80x9d) is proximal to a disk surface, and can be scanned to select a radial position on the disk. The disk spins rapidly beneath the (effectively stationary) head. This enables rapid reading or writing of information on nearly circular bit tracks that cover most of the disk surface. Some types of storage media, such as DVD and CD disks, can be recorded in advance by injection molding the disk with a permanent bit sequence, so that a read-only (ROM) disk is produced.
A common DVD-ROM format stores a series of files or data sequences as a continuous data string. FIG. 1 illustrates such a data string 110, which includes a first data sequence 111 followed without delay by a second data sequence 112, a third data sequence 113, and others not shown. This format is convenient for reading of pre-recorded data sequences (i.e. files).
Bits may also be recorded by a head, on re-writable media. With re-writable media, it is desirable to be able emulate the DVD-ROM format, and also be able to replace a specified data sequence within a data string, such as second data sequence 112; with another data sequence, such as new second data sequence 122. The desired result of this edit is edited data string 130. Edited data string 130 has old second data sequence 112 replaced by new data sequence 122, while data sequences 111 and 113 are unchanged.
Unfortunately, the DVD-ROM format has no mechanism to accurately time the beginning of the over-write process for an edit. Thus, there is often a timing error, xcex94t, in the writing of a new second data sequence 122. The data sequences pass rapidly beneath the head, at a velocity, v, that may be radius dependent and is given by the following equation.
v=x/txcx9c3.5-8.8 m/s,xe2x80x83xe2x80x83(1)
where xe2x80x9cxxe2x80x9d is the distance along the track and xe2x80x9ctxe2x80x9d is time. Thus, the timing error, xcex94t, corresponds to a position error, xcex94x, given by the following equation.
xcex94x=v*xcex94t.xe2x80x83xe2x80x83(2)
When the write process for new data sequence 122 begins too early, it writes over part of first data sequence 111, as shown in data string 140. Furthermore, part of old second data sequence 112 remains. Similarly, when the write-timing of new data sequence 122 is delayed, the beginning of third data sequence 113 is erased and the beginning of old second data sequence 112 remains, as shown in data string 150.
Thus, the standard DVD-ROM format has the disadvantage that it does not accurately edit data sequences. The edited data sequence is not completely replaced by the new data sequence, and nearby data sequences can be damaged or erased. There is a strong motivation to design a re-writable DVD format that emulates the DVD-ROM format because then the existing installed base of DVD players and DVD-ROM readers can read the re-writable DVD disk. Hence, there is a need in the art for a method to accurately synchronize new edited data with the existing data on the disk. This method should accurately position recorded data sequences in order to repeatably write and read information. This method should (1) precisely control the time and distance between the data sequence start positions and well defined locations on the disk, (2) precisely position and accurately calibrate the beginning of the write process with the data sequence start position, (3) consistently calibrate the beginning of the write process, independent of which recording device is used, and (4) compatibly operate with prior DVD technology.
This disclosure is directed towards a method and apparatus for calibrating write timing of recorded information.
The calibration involves (1) writing a calibration data sequence referenced to a timing clock including positions marked on the disk (such as a wobble clock on a DVD disk), (2) measuring the timing offset or phase offset between the calibration data sequence and the clock, (3) writing a test data sequence using the same or a different recording device, (4) measuring the timing offset or phase offset between the test data sequence and the clock, (5) comparing the timing offsets of the calibration and test data sequences, with respect to the clock, (6) Iteratively adjusting the timing of the beginning of the write process, and repeating steps (4) and (5) until the timing offsets are equal to within a defined tolerance, and (7) Adjusting the delay of the write head relative to the clock by the amount determined in step 6.
This process calibrates the timing of bits written on DVD+RW disks by different recording devices so that the start position of edited data sequences is repeatably collocated with previous data sequences to a precision better than +/xe2x88x920.06 xcexcm.
These and other objects, features, and advantages of the present invention will be apparent from the included drawings and from the following detailed description.